1. Field of The Invention
This invention relates to a semiconductor device having field-shield isolation structures and especially, adapted to be used for a semiconductor memory device such as DRAM (dynamic random access memory), EEPROM (electrically erasable programmable read only memory). It also relates to a flash memory and a method of making such a semiconductor device.
2. Description of The Related Art
There are known hitherto, as the element isolation technique, a method of local oxidation of silicon (LOCOS) as disclosed, for example, in JP-A-5-136429 and a trench isolation method as disclosed, for example, in JP-A-5-198778 and "A 3.6 .mu.m.sup.2 Memory Cell Structure for 16 MB EPROM" Y. Hisamune et al, IEDM-89, 1989, p.p. 583-586.
When it is desired, however, to increase the integration density of LSI, the LOCOS method involves problems such as generation of bird's beaks and space limitation in element-isolation due to the required thickness of the element-isolation oxide film. It also has a problem concerning electrical property such as field inversion due to lateral diffusion of impurities and short channel effects. On the other hand, the trench method requires complicated processes for forming trenches in a semiconductor substrate.